1. Field of the Invention
The present invention relates to a cellulose acylate composition which contains a very small amount of fine impurity and is useful for an optical film, and a method for producing the same. Furthermore, the invention pertains to a high grade optical film, a retardation film, a polarizing plate, an optical compensation film, an anti-reflection film, and an image display device employing the above-mentioned cellulose acylate.
2. Description of the Related Art
Due to the transparency, toughness, and optical isotropy, cellulose acetate is increasingly finding its usefulness in a variety of applications, including the use in the support of photographic sensitive materials, as well as the use in optical films for image display devices including liquid crystal display devices and organic EL display devices. With regard to the optical film for liquid crystal display devices, methods of using cellulose acetate for polarizing plate protective films, or for retardation films for liquid crystal display devices of STN (Super Twisted Nematic) mode or the like by stretching the film to attain in-plane retardation (Re) and retardation in the thickness direction (Rth), are being implemented.
In recent years, there have been developed display devices of VA (Vertical Alignment) mode, OCB (Optical Compensated Bend) mode, or IPS (In-Plane Switching) mode, where higher values of retardation such as Re and Rth are required compared with the STN mode. Thus, an optical film material having property of manifesting various types of retardation according to the type of liquid crystal mode is on demand.
Stretchability of cellulose acetate is poor, and an area where retardation due to stretching and alignment of only polymer is realized is limited. Furthermore, since cellulose acetate is a relatively hydrophilic polymer, a change in retardation caused by humidity is significant.
In order to cope with such demand, a cellulose acylate film has been disclosed as a new material for optical film, which is produced by a solution casting method in which a solution of mixed esters of an acetyl group and a propionyl group of cellulose (mixed acylate of cellulose such as cellulose acetate propionate) is flow cast on a support, a portion of the solvent is evaporated, and then a cellulose acylate film is peeled off from the support (see JP-A-2001-188128). In addition, there is a process in which cellulose acetate butyrate and cellulose acetate propionate are used as mixed acylate of cellulose that has a melting temperature lower than that of cellulose acetate and then melt cast to form an optical film (see JP-A-2000-352620). The melt casting has the following advantages. Since an organic solvent is not used during the casting, a dissolution or dry process may be omitted unlike the solution casting, and a load to environment is low.
Mixed acylate of cellulose such as cellulose acetate propionate and cellulose acetate butyrate is an excellent material that increases retardation of cellulose acetate. Meanwhile, in the case of when a process where cellulose and acid anhydrides that are industrially available are reacted in the presence of an acid catalyst to produce cellulose acylate is used, reactivity is lower as compared to cellulose acetate, impurities including unreacted cellulose may easily remain. The impurities are observed as black point impurities or bright point impurities under a crossed Nicols condition. In the case of when cellulose acylate is used as an optical film, optical defects may be formed or light leakage may occur, thus it is required that the amount of impurities is set to be very small.
An activation process in which an acetic acid is added to raw material cellulose and a temperature is maintained at 40° C. for 1 hour or more is disclosed as a process of reducing unreacted cellulose of mixed acylate of cellulose (see JP-A-2006-45500).
This process is useful to reduce the amount of unreacted substances while the degree of polymerization of cellulose acylate is maintained at a relatively high level. However, it is difficult to remove impurities other than unreacted cellulose, and when the required amount of unreacted substance is very small, it is necessary to perform a process of removing the impurities.
In the case of when cellulose acylate is used as a raw material of the optical film, if the casting process is the solution casting process, even though cellulose acylate containing a large amount of impurity is used, a cellulose acylate solution is prepared, filtered by using a filter having a small retention particle size, and cast to significantly reduce the amount of impurity of products (see JP-A-2003-213004).
Furthermore, a process of filtering melts of cellulose ester in the case of when the melt casting is performed is disclosed. In the process, desirable filtration precision is 5 μm or less (see JP-A-2000-352620). However, there is a problem in that replacement of a filtering material during the filtration of the melts is more difficult as compared to the filtration of the solution.
Therefore, in order to produce cellulose acylate useful to provide acceptable optical properties when the melt casting is performed, it is required that the amount of fine impurity contained in the raw material of cellulose acylate used to perform the melting is set to be very small to make the filtration of melts unnecessary. However, this is difficult to be achieved by using a known method, thus there is a need to provide a method of solving this problem.